Effect

Generation III

Focus Punch is a decreased priority move. The user of Focus Punch will charge up during the beginning of the turn it is executed, then attack at the end unless it is hit by another Pokémon's attack before launching its own; in which case it will lose its focus and be unable to attack that round.

Charging turn has a priority of +8.

Generation IV and on

Charging turn has a priority of +6.

Pokémon Mystery Dungeon

Focus Punch has a charge-up turn (akin to Skull Bash, Sky Attack, etc.), and can hit even when the user is hit during the turn of charge-up, unless the user is inflicted with a Status ailment like paralysis or sleep.

Trivia

Since Focus Punch has a higher priority than Avalanche, Revenge, Counter, Dragon Tail, or Circle Throw, they are unable to break the focus of a Pokémon using Focus Punch. Thus, they are the only attacks that can strike a Pokémon in the same turn it successfully uses Focus Punch under normal circumstances.

In Generation III, a glitch will cause the user of Focus Punch to prepare to use it, but not execute the attack. This glitch is triggered by selecting Focus Punch in a Double Battle on the first Pokémon, pressing the "B" button, then switching out the first Pokémon. When the battle is conducted, the first Pokémon will tighten its focus in preparation for Focus Punch, then switch out. This is due to charging turn having a priority of +8, and therefore being executed before switch out.

Shroomish and Togekiss are able to learn Focus Punch, despite lacking any visible hands to use the move. In Shroomish's case, it is likely explained by the fact that it evolves into Breloom, which not only has arms, but also is a Fighting type. The fact that Togekiss can learn the move is likely due to the fact that it evolves from Togetic, a Pokémon that does have hands.